Recuperators



Nov. 7, 1961 J. D. KELLER 3,007,631

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United States atent 3,007,681 RECUPERATORS John D. Keller, Farmers Bank Bldg, Pittsburgh, Pa. Filed Oct. 4, 1957, Ser. No. 688,203 4 Claims. (Cl. 257313) This invention relates to new and useful improvements in recuperator structures and more particularly to metal recuperators and the regulation and control of the cold air entering the recuperator in a manner to prevent overheating and burning out of the metal tubes.

The present application is a continuationin-part of an application serially numbered 507,790, filed May 12, 1955, and now Patent No. 2,947,522. In the former application, I have disclosed means for effectively maintaining safe temperatures in the metal tubes of recuperator structures by regulating the air velocity in the recuperator when the latter is operating at less tthan its maximum rate.

In accordance with the present invention, control means other than those disclosed in the former application are employed for maintaining safe metal temperatures and flow controls are provided for applying the invention to the counterflow type of recuperator.

Further objects of the invention will become apparent from a consideration of the accompanying drawings constituting a part hereof in which like reference characters designate like parts and in which:

FIGURE 1 is a vertical longitudinally sectional view through a suspended tube recuperator;

FIGURES 2, 3 and 4 are side elevational views of a portion of the air supply flow and duct with flow control valves and operating mechanisms embodying the principles of this invention;

FIGURE 5, a vertical longitudinal sectional view through a tubular recuperator structure of the counter flow type; and,

FIGURE 6, a vertical sectional view through a stack type recuperator.

As shown in FIGURE 1, the hot gases coming from the furnace enter the recuperator chamber at 1 and exit at 2 wiping the outer Walls of the banks of suspended metal tubes 4, 5 and 6. The banks of tubes 4, 5 and 6 are provided with headers 4a, 5a and 6a, respectively, which are interconnected by ducts 7 and 8. Headers 4a and 6a are respectively connected to ducts 10 and 11 leading from a blower duct 9 through which cold air is supplied by fan or blower 3. Header 5a is connected by a hot air duct 5b for delivering all of the preheated air to the furnace burners (not shown).

The banks of tubes have inner tubes as shown in section at 612, FIGURE 1. The inner tubes are connected in the upper header chamber 60 and receive the cold air from duct 11. The cold air passes down inside the inner tubes 6b and from the open end 6d at the bottom into the space between the inner tubes 6b and outer tube 6. It becomes heated as it passes upward to the bottom header chambers from which it goes to the upper header chamber So then down the inner tubes and up between the inner tubes and outer tubes 5 to chamber 5a from which the preheated air passes to use through duct 51).

Cold air is supplied to the banks of tubes as shown by arrows in ducts 9, 10 and 11. When the furnace is operating at less than maximum capacity, the air flow is reduced. This may be accomplished in various ways as follows. The numeral 12 designates a valve in duct 11. The numeral 13 is a valve in the supply duct 9.

Valve 12 is controlled by a cam plate 14 actuated by a hydraulic or pneumatic cylinder 15 (or alternatively by an electric operator or power unit), which in turn is controlled by the ratio-control device of the furnace.

This device is not shown, but may be of any of the standard types. In all furnaces of the types to which these designs of recuperators are applied, the rate of fuel supply to the furnace is either set manually by the operator or automatically by the temperature controller on the furnace; the ratio controller on the furnace then varies the rate of air flow to the furnace to suit the fuel rate. Thus the ratio controller forms part of the furnace instrumentation and would be present whether a recuperator were used or not. The difference lies in the manner in which the air supply valves are arranged and operated, in the present invention.

In FIGURE 1, when cam plate 14 is moved by the hydraulic cylinder 15 to its extreme left position, both air-valves 12 and 13 are wide open and the furnace is receiving maximum air from the recuperator. As the ratio controller of the furnace calls for less and less air, the cylinder moves the cam plate gradually to the right, first closing valve 12 and then later closing valve 13.

In the position shown, the sloping part 18 of the cam has moved roller 17 on lever 16 to turn valve 12 to the almost closed position. (Valve 12 is never entirely closed because it is desired to have at least some air flow through tubes 6 at all times.) As the cam moves further to the right, roller 17 travels in slot 19 of the cam which is parallel to the direction of motion and merely holds valve 12 in the nearly closed position.

Valve 13 in the main air supply line is normally held wide open by spring 23 holding its lever against stop 22. As the cam plate 14 moves still furtther to the right, its end plate 21 contacts roller 21 and moves the lever to gradually close valve 13.

Thus as the total air flow is reduced, the flow through the cold end pass of the recuperator including tubes 6, is reduced while the flow through the hot end pass includ ing tubes 4, is maintained at the full rate in order to protect the metal of the tubes. Only after the valve 12 has been closed to its limiting position is valve 13 moved to reduce the air flow to the entire recuperator.

When the ratio controller of the furnace is of the electrical type the arrangement of new FIGURE 2 is used and valves 12 and 13 are controlled by the electric operators or power units 24 and 25, which contain reversing motors that can remain in a stalled po sition and are a standard article of manufacture. Lead Wires 24a connect a power circuit from the furnace ratio controller (not shown) to both units 24 and 25, through switches 26 and 29. In the position shown, both valves are Wide open and the air fiow through the recuperator is at a maximum, While switch 26 is open and switch 29 is closed, hence unit 24 is connected for operation while the circuit to unit 25 is open.

When the ratio controller calls for a reduction or" the air flow, unit 2-iturns its shaft clockwise and, through its lever system, closes valve 12 which supplies air to the cold end pass of the recuperator, tubes 6. After valve 12 has been closed as far as it is intended to go (not entirely closed, for the reason previously mentioned in column 2) the lever of valve 12 has closed switch 26 completing the circuit through unit 25. While the motor of unit 24 remains stalled in its extreme clockwise rotated position, unit 2 then turns its shaft clockwise and, through the lever system, closes valve 13; at the same time, the clockwise motion of the operator lever allows compression spring 31 to draw the contacting blade of switch 29 away from contact 319 and thereby break the circuit to unit 24-.

When the ratio controller again calls formore air, it causes unit 25 to turn its shaft counterclockwise, opening valve 13 and, as it approaches its extreme counterclockwise-rotated position, closing switch 2% to complete the circuit through unit 24 and cause the latter to rotate counterclockwise, opening valve 12.

In new FIGURE 3, the hydraulic or pneumatic cylinder 15 controlled by the furnace ratio-controller moves cross-head 32 which slides in guides 32a and has connected to it rods 33 and 35, which are jointed respectively to lever 34 operating valve 12 and lever 36 operating valve 13. The proportions and angular positions of these links and levers are such that when the ratio controller calls for reduction of the air supplied to the furnace and causes cylinder 15 to move cross-head 32 progressively toward the right, the linkage of valve 12 having then a small lever arm moves quickly in the closing direction, while the linkage of valve 13 having then a large lever arm moves only slowly in the closing direction. Later, as cross-head 32 approaches its extreme righthand limit, the lever arm of valve 12 has become large while the lever arm of valve 13 has become small; valve 12 then moves only slowly while valve 13 moves rapidly in the closing direction.

In new FIGURE 4, the cylinder 15 controlled by the furnace ratio-controller moves (both together) cams 39 and 40, which act on rollers 37 and 38 to close or open valves 12 and 13, respectively; these are here shown as slide valves but may be of the butterfly or any other suitable type. When the cams are in the extreme leftward position, both air valves 12 and 13 are wide open and the air flow through the recuperator is at its maximum. Cam 3% is so located, with respect to cam 40, that as the cylinder moves the cam assembly to the right, the sloping part of cam 39 engages roller 37 before the sloping part of cam 40 engages roller 38. When the ratio controller calls for a reduction of air flow, cylinder 15 moves the cam assembly progressively toward the right. Cam 39 first closes valve 12; in the position shown, valve 12 has been closed about half-way. After it has been closed as far as intended (see previous note, column 2), roller 37 merely rides in the part 39a of the cam slot which is parallel to the direction of motion and hence causes no further motion of valve 12; while the sloping part of cam 40* engages roller 38 and closes valve 13.

Although cams 39 and 4% move as a unit, cam it is preferably made shiftable (in the direction of motion) with regard to cam 39, so that the closings of valves 12 and 13 may be made to overlap somewhat in cases where this is desired.

From all of these FIGURES l to 4, it will be evident that valve 12 may be placed in the outlet duct 8 instead of in the inlet duct 11 to the cold end pass including tubes 6; and/ or valve 13 may be placed in the main hot air duct from the recuperator to the furnace instead of in the main cold air supply duct 9. In the latter case, if a bleeder valve is used, such as valve 47 in FIGURE 6, I preferably locate valve 13 in the hot air duct beyond the bleeder outlet, i.e., between said outlet and the furnace.

It will also be evident that valve 13, instead of being located in the main air duct 9, may be located in the cold air duct Till leading to hot end pass including tubes 4, or may be located in the warm air outlet duct 7 from this pass; the valve operating mechanism of any type being so arranged that valve 12 is closed first and valve 13 later (although some overlapping may be desired in some cases). It will also be evident that instead of a hydraulic cylinder or an electric power unit actuated by a furnace ratio-controller, a manually operated lever or handwheel may be used for varying the air flow.

FIGURES and 6 apply to counterflow recuperators. The multi-tube type may be arranged with all tubes in series as in FIGURE 5, or divided into groups of tubes in parallel with two or more passes in series, corresponding to tube banks 6 and 5 in FIGURE 1.

In FIGURE 5, the cold air duct 9 from the blower 3 has two branches, one designated 10, supplying air to the tubes in the cold end of the recuperator and the other, designated 11, connected to a point 41 partway along the path of air flow through the recuperator. Valve 42 located in duct 11 is normally closed. The hot air passes from the recuperator through outlet duct 46 to the furnace. 43 is an electric operator or power unit connected by rod and lever linkage to valve 42 and controlled by the automatic temperature controller 4-% having thermocouple 45, which thermocouple may be located either (as shown) in the air outlet to measure the temperature of the outgoing air, or may be welded or otherwise suitably attached to the hottest tube 49 to measure the maximum metal temperature.

47a is a bleeder outlet from hot air duct 46, having in it a valve 47 actuated through rod and lever linkage by electric power unit 43.

Temperature controller 44 is preferably of the twolimit-point type (which is a standard article of manufacture). When the temperature indicated by thermocouple 45 rises and reaches a certain set point corresponding on the temperature scale of the instrument 44 to contact 49, the pointer 44a makes contact with 49 and actuates power unit 43, causing the shaft of the latter to turn counterclockwise, whereby to progressively open valve 42 and admit cold air at 41 to the hot end of the recuperator, reducing the metal temperature between 41 and 49 and especially that of the hottest tube 49.

It is obvious that the valve 42, instead of being actuated by the temperature of the heated air leaving the recuperator, may alternatively be controlled by the temperature of the metal of the tubes at their hottest point, as 49 (by welding thermocouple 45 to the tube at that point); or by the rate of air flow of the differential air pressure derived therefrom between points 41 and 46 as in my co-pending application. It is also apparent that valve 42 could be placed in duct 10 instead of 11 and arranged to be normally open and to close as the temperature measured by thermocouple 45 rises.

If the temperature of thermocouple 4 5 rises still further, pointer 44a touches contact 5t which is set at a higher temperature than 49, and actuates power unit 48 in a clockwise direction to open bleeder valve 47 and discharge hot air to the atmosphere, thereby increasing the total flow of cold air into the recuperator and further reducing the metal temperature.

In FIGURE 6, the counterilow recuperator is of the stack type. The hot waste gases from the furnace enter at I, pass upward inside the cylindrical heat-conducting Wall 51 and pass out to atmosphere at '2. Air supplied by a blower (not shown) through ducts 9 and Ill enters at the top and flows downward between said wall 51 and outer shell 52.

A branch duct 41 from main cold-air supply duct 9 has in it a valve 42, and connects to the recuperator air passage at 41a, which preferably is about /3 to /2 the height of 51 from the top. In the hot air outlet duct 46 is located a valve 46a.

A hydraulic or pneumatic cylinder 15, controlled by the furnace ratio-controller, actuate-s dual cam 3940 to close or open valves 42 and 46a. In the position shown, the furnace ratio-controller has called for a reduction of air flow, and cylinder 15 is moving the earn assembly toward the right. Cam 4t) engaging roller 38 has already closed valve 46a halfway, and further motion to the right will cause cam 39 to act on roller 37 to progressively open valve 42 while cam 4% continued to close valve 46a. In this way, as the total air flow is reduced, a progressively 1 greater proportion of the air is by-passed around the top part of the recuperator and contacts only that part of the heat-transmitting wall 51 between 41a and the outlet, thereby reducing the air temperature and correspondingly the metal temperature, especially at the hottest part, as 51a.

Cams 39 and 40 move as a unit but are preferably made adjustable in the direction of motion with respect to each other, so that the degree of overlap of the motions of valves 42 and 46a may be varied.

In starting up a furnace, it is often necessary to use 25% to 30% more combustion air temporarily than will be used by the furnace at full operating rate after it has come up to temperature. The fact that the air passing through the recuperator is at a lower temperature during starting up automatically provides some increase of air flow capacity. But because of the drooping characteristic curve of the types of blower ordinarily provided for supplying the air, this increase is far from sufficient. If the air passages in the recuperator are proportioned correctly for the quantity of air required in normal operation at full load, then since the presure drop increases about as the square of the velocity, a blower must be provided capable of producing about 50% higher pressure than would otherwise be required. This requires an excessively large and expensive motor and blower resulting in ineffioient operation under normal conditions. If, on the other hand, the air passages in the recuperator are made large enough to pass the maximum volume of air required when starting up with only the normal pressure drop, then for operation after the furnace is heated up, the air velocity in the recuperator will be too low for good efficiency and for proper protection of the metal of the recuperator.

To avoid these difliculties, I provide a cold air by-pass 53 around the recuperator as shown in FIGURE 1, from cold-air duct 9 to hot-air duct 5b, and in duct 53 I provide a regulating valve 54. Preferably I either make duct 53 so small, or provide in it a restriction of such limited area, that even with valve 54 wide open, no more than about 30 percent of the rated normal maximum air volume can pass through it.

In my co-pending application Serial No. 507,790 I showed the valve in the by-pass duct as being manually operated. The trouble with this is that the furnace operators forget to close the by-pass valve after the furnace is up to operating temperature. I therefore preferably arrange to operate the valve 54 automatically, as shown in FIGURE 1, for example, by means of a solenoid 55, the lead wires 56 of which connect to a source of electric current and to a switch '57. This switch is normally held open by a spring (not shown), but when the demand is for the maximum possible flow to the furnnace and cylinder 15 moves cam plate 14 to its extreme left position, the left end of 14 touches the lever of switch 57 and closes the circuit through solenoid 55 which opens valve 54 in the by-pass duct 53. When the demand for air decreases and cam-plate 14 moves slightly toward the right, switch 57 opens the circuit so that solenoid 55 is no longer energized, and spring 58 then closes bypass valve 54.

Alternatively, pneumatic or hydraulic means may be used to open valve 54 when the demand for air reaches a set limit and to close the valve when the demand falls below this limit. It is also obvious that instead of switch 57 being operated by cam-plate 14, it may be actuated either by the fuel demand controller if one is used on the furnace, or by the cylinder or actuator which opens or closes the inlet control vanes (not shown) on the combustion air blower 3 if the air fiow is controlled by such 6 vanes instead of by dampers in the ducts connected to the outlet of the blower as shown in FIGURES 1 to 6.

I claim:

1. In a recuperator for an industrial heating furnace, a hot gas chamber for receiving the products of combustion of said furnace having heat exchange conduits for heating air disposed in the path of travel of the hot gases passing through said chamber, said conduits being divided into a plurality of air passes each having air inlets and outlets, the conduits of one of said passes being disposed in the region of the hottest gases passing through said chamber, the conduits of another of said passes being disposed in the region of the coolest gases in said chamber, and the conduits of the other of said air passes being disposed intermediate the second and first-named passes, an air duct leading from a source of air connected to the inlets of the conduits of said first and second-named passes, means connecting the air outlets of the first and second named passes to the air inlet of the other pass, said air duct having a valve and said second air pass having a valve, an air flow controller for varying the volume of air supply with the heat demands of the furnace and means controlled by the air flow controller to actuate said valves to reduce the total air flow to said passes.

2. A reouperator structure as set forth in claim 1 in which the valve actuating means is operative to first close the air valve in the second pass and then close the air valve in the air supply duct.

3. A recuperator structure as set forth in claim 1 in which the valves are operated by a double acting cam consisting of two parts one part controlling the air valve in the second pass and the other part controlling the air valve in the air supply duct, said parts of the cams being adjustable relative to each other to vary the overlap of the motions of said valves.

4. A recuperator as set forth in claim 1 in which the air flow controller actuates means for reducing the air flow through the second pass to maintain substantially constant air flow through the first pass when the total air flow is reduced.

References Cited in the file of this patent UNITED STATES PATENTS 1,599,613 Fahrenwald Sept. 14, 1926 1,633,759 Breese June 28, 1927 1,870,809 Handy Aug. 9, 1932 1,913,684 Purdy June 13, 1933 1,927,215 Peebles Sept. 19, 1933 1,964,256 Fahrenwald June 26, 1934 2,511,647 Marshall June 13, 1950 2,551,697 Palmatier May 8, 1951 2,729,301 Ekstrom Jan. 3, 1956 2,795,401 Cooper et al. June 11, 1957 FOREIGN PATENTS 232,209 Switzerland Aug. 1, 1944 

